Short answers, pulled from the story.
The Red Army was established on the 28th of January 1918 by a decree of the Council of People's Commissars. It was created to defend the new Bolshevik government against adversaries in the Russian Civil War, particularly the various anti-Bolshevik groups collectively known as the White Army, after the Imperial Russian Army had collapsed.
Up to 34 million soldiers served in the Red Army during World War II. This total included 29,574,900 men conscripted during the war added to the 4,826,907 already in service at its start. Around 8 million of the 34 million mobilized were non-Slavic minority soldiers.
The official total of Red Army dead in World War II was 8,668,400, comprising 6,329,600 killed in action, 555,400 deaths by disease, and 4,559,000 missing in action. Other estimates place the total dead at nearly 11 million, and the Russian Central Defense Ministry Archive database lists names of roughly 14 million dead and missing service personnel.
The Great Purge of 1937-1939 removed 3 of 5 marshals, 13 of 15 army generals, 8 of 9 admirals, and 154 of 186 division generals from the Red Army. The resulting inexperience among senior officers left only 5% or fewer of army and military district commanders with two or more years of command experience by June 1941, contributing directly to the Red Army's catastrophic early defeats against the German invasion.
The Red Army launched the Soviet invasion of Manchuria on the 9th of August 1945, three months after Germany's surrender as agreed at the Yalta Conference. Soviet forces, supported by Mongolian troops, overwhelmed the Japanese Kwantung Army and drove into Manchukuo, Mengjiang, and the northern portion of Korea. Emperor Hirohito announced Japan's surrender on the 15th of August 1945.
The deep operation doctrine was developed in the late 1920s and 1930s under Marshal Mikhail Tukhachevsky, drawing on lessons from the Polish-Soviet War and Russian Civil War. It called for simultaneous corps- and army-size maneuver attacks striking throughout the full depth of enemy defenses, combining aviation and armor. The doctrine was first formally expressed in the 1929 Field Regulations and codified in the 1936 Provisional Field Regulations (PU-36), but was abandoned after Tukhachevsky's execution in the purges and only applied during the Second World War.